SWMM 5 Link Iterations

These three graphs show how the number of iterations to solve the St. Venant equation in SWMM 5 changes during the course of the simulation based on rapidly changing inflow, steady inflow and decreasing inflow. This example allows up to ten iterations and a tighter head tolerance to better illustrate how the number of iterations increase at the beginning of the simulation and during rapid inflow. Normally, in SWMM 5 the number of iterations will be between a minimum of 2 and a maximum of 4 iterations.

In the first graph the outflow is in blue and the number of iterations at each time step is shown in red. In the second graph the bubble size is based on the number of iterations and the y axis is outflow of the network. The third graph shows the number of iterations used at each link in the model at a particular time step. The more the flow changes the more iterations are needed to keep the flow in balance.

SWMM5 Bubble Plot of Continuity Error

The overall continuity error at any time during the simulation is simply the total inflow minus the total outflow. The total inflow is the dry weather, wet weather, groundwater, I&I inflow, external inflow and the initial network storage. The total outflow is the amount of surface flooding, outfall flow, reacted flow and the final storage.

Continuity error = Total Inflow - Total Outflow

The continuity error can be variable over time as this graph of the total inflow, total outflow and continuity error over time shows for the classic extran example from SWMM 3 and SWMM 4. The continuity error can be negative or positive at each saved time step and it tends to balance out over time. As you can imagine depending on how long the simulation lasts the continuity error may be much greater than zero. If you can the simulation to dry weather flow is reached in the sanitary network or the stormwater network has drained the continuity error will be better. You can see that the CE increases at the beginning of the simulation, continues on and then goes to zero CE when the system drains.

If we look at a bubble chart of the continuity error over time (with the bubble size the continuity error) and the y axis the Total Inflow to the network you can see how continuity error increases and then decreases over time. The white bubbles are negative continuity error points.

SWMM 5 Conduit Lengthening

If you use the conduit lengthening option under the dynamic tab the shorter lengths will be lengthened internally during the simulation and the results will be a smoother.